The present invention relates to the field of mass spectrometry and in particular to a differential ion mobility separator.
The mobility K of an ion in a gas in the presence of an electric field E is essentially independent of the field under conditions where the energy gained from the field by the ion is negligible compared with thermal energies. This condition is met when E/N is low, wherein N is the neutral gas number density. However, as the electric field is increased or the neutral gas number density is decreased, then the mobility of an ion becomes dependent on the ratio of E/N:
                              K          ⁡                      (                          E              N                        )                          =                              K            0                    ⁡                      [                          1              +                              α                ⁡                                  (                                      E                    N                                    )                                                      ]                                              (        1        )            wherein K0 is the low E/N mobility and α(E/N) is a function representing the field dependence.
A knowledge of the dependence of the mobility of an ion on electric field strength prompted development of a differential ion mobility analyser by Buryakov et al. (International Journal of Mass Spectrometry and Ion Processes, 128 (1993), pp 143-148) which disclosed separating ions according to differences between their low and high field mobilities.
The device disclosed by Buryakov et al. consisted of a pair of parallel plates with a gas flow between them. Ions to be separated were entrained in the gas flow.
Known differential ion mobility analysers operate at atmospheric pressure. At atmospheric pressure the rate of ion diffusion is lower than at reduced pressure and so as the gas pressure is reduced, ion diffusion becomes a significant loss mechanism. Conversely, at atmospheric pressure high voltage RF generators are necessary in order to provide values of E/N which are high enough to access high-field mobility conditions.
In known differential ion mobility devices ions having specific differential ion mobility characteristics may be transmitted sequentially as a result of scanning the operating parameters of the device. Ions which have entered the device, but which are not being transmitted, will be lost. This results in very low duty cycle for transmission of analytes having differing ion mobility characteristics during an analytical scan and hence low overall sensitivity.
It is desired to provide an improved differential ion mobility separator and method of separating ions according to their differential ion mobility.